One process of producing polysilicon material for use in various industries involves “growing” polysilicon onto a base of high purity polysilicon. For example, the Siemens process or similar chemical vapor deposition (CVD) processes are used to produce approximately 80% of the high purity polysilicon produced today. In the Siemens process, polysilicon cylinders (sometimes called boules) are produced by the pyrolytic decomposition of gaseous silicon compound onto a silicon substrate (sometimes referred to as a slim rod). In these processes, polysilicon material is grown in a chemical reactor by means of chemical deposition onto a core material of polysilicon at an elevated temperature. Typically, the core material is initially formed into a U-shaped filament, sometimes called a hairpin, which is enclosed by a pressure vessel and is heated, at least in part, by passing current through it. Gas is fed into the pressure vessel and additional material is deposited onto the filament by Chemical Vapor Deposition (CVD) until it grows substantially in cross-section and surface area eventually forming an approximately cylindrical cross-section of the desired diameter and is removed. The resulting built up filament is cut into cylindrical sections (referred to as a boule). The resulting material is intended to be ultra-pure silicon that is used as feedstock for subsequent processes including the production of semiconductor and photovoltaic wafers. Hairpins are typically produced by joining long thin strips of polysilicon, known as slim rods, and a cross-link. Slim rods may be produced in several alternative ways.
There are modified processes for producing polysilicon that do not require a hairpin electrode, which instead of a ‘bell jar’ like chamber use a tubular chamber and linear filaments. These approaches still require the production of slim rods. Further alternatives avoid the need for slim rods altogether but none so far have proven to be as effective as the Siemens process or similar processes.
Slim rods may be produced in several alternative ways. In some cases, slim rods are produced by recycling some of the reactor output. Boules are cut into slabs and subsequently cut into thin strips (the slim rods) which are joined together to form new filaments. Conventionally, saw cutting is used to manufacture silicon slim rods. However, this method typically produces a large kerf resulting in reduced material utilization. Saw cutting also tends to have a low feed-rate resulting in low throughput and the need for a considerable amount of equipment to support a large-scale plant. Saw cutting is also a contact method with the inherent possibility of contamination by abrasives and other materials, which is a potential liability in production of high purity material. Saw cutting is also prone to cause breakage, which leads to reduced process yield. Slim rods are typically 6 to 12 millimeters on a side while saw kerfs are typically 1.2 to 1.5 mm. While the kerf material can be recycled, this still represents a substantial loss of valuable material. In some cases, saw cut kerf loss is approximately 18% (1.2 mm kerf versus a 6.5 mm wide slim rod). Typically, chemical etching or other processes also need to be used to clean the slim rods after saw cutting. Approximately 1 out of 48 boules produced are cut into slim rods for re-use in the growth system.
Two parameters, which are somewhat opposing, are the need to make hairpins as long as possible and as thin as possible in order to maximize both material utilization and reactor chamber utilization. As rods become longer and thinner, it becomes more difficult to cut them without breakage and, of course, kerf loss is accentuated (that is, becomes a larger fraction of the polysilicon consumed).
In an alternative, diamond wire saws may be used. Diamond wire sawing can reduce the kerf loss to under 0.2 mm but has other limitations. For example, diamond wire sawing is typically a slow process: wire speed and material removal rates are limited as is cutting force and thermal management of the wire is also critical. A typical cutting speed is in the order of mm/min, consequently cutting the length of a slab, typically up to 2.3 m in length, can be quite time consuming. Additionally, wire cutting without a coolant typically proceeds more slowly creating a tradeoff between use of coolant and reduced feed rate, neither of which is desirable.
In an alternative process, slim rods may alternatively be produced by pulling silicon from a crucible; however, this is a slow and energy intensive process. Also, the resulting rod profile, which is commonly cylindrical, makes assembly of slim rods into a hairpin filament, for example by butt welding, more difficult.
It should also be remembered that tool wear is a problem with cutting tools (disk, wire, grit, etc) as cutting tools typically include items requiring frequent replacement.
In general, the cost of production of slim rods is a significant cost factor in the production of high purity polysilicon. There is a need to provide an improved system and method for production of slim rods that may provide at least one of the following: increased equipment capacity, reduced floor space, reduced energy use, reduced scrap and consumables, among other things, in order to reduce cost of manufacture.
In examining an improvement, material utilization is a useful parameter: the greater the number of slim rods that can be produced from a single boule, the fewer the number of boules that are required to sustain the process, consequently a greater amount of polysilicon is output for a given reactor capacity. As noted above, factors that affect material utilization are kerf loss and breakage and slim rod dimensions.